I was wondering if it was possible to setup a conditional http basic auth requirement based on the virtual host URL in an .htaccess file.
For example what I want to do is have mysite.com and test.mysite.com run off the same code base in the same directory but password protect test.mysite.com. It would be setup this way so that I wouldn't need to branch my code since my app code can see which vhost/url it's being served from and pick the database to serve content from.
You can sort of kludge this by using mod_setenvif along with the mod_auth modules. Use the SetEnvIfNoCase directive to set which host is password protected. You'll need a couple of extra directives to satisfy access:
# Check for the hostname here
SetEnvIfNoCase HOST ^test\.mysite\.com\.?(:80)?$ PROTECTED_HOST
Then inside the Directory block (or just out in the open) you have your auth stuff setup, something like this:
AuthUserFile /var/www/test.mysite.com/htpasswd
AuthType Basic
AuthName "Password Protected"
Now for the require/satisfy stuff:
Order Deny,Allow
Satisfy any
Deny from all
Require valid-user
Allow from env=!PROTECTED_HOST
This will make it so any host that doesn't match ^test\.mysite\.com\.?(:80)?$ will have access without need for auth (Allow from env=!PROTECTED_HOST) but otherwise, we need a valid user (Require valid-user). The Satisfy any ensures that we just need one of the 2, either the Allow or Require.
I had problems implementing Jon's solution: 
Although I am quite familiar with Apache conf and regular expressions, the authentication always fired. From a quick analyzes it looked like the Allow from env=!PROTECTED_HOST line did not kick in.
But I found another solution that actually looks safer to me:
I created two virtual hosts for the two domains pointing to the same document root (which is fully allowed by the way). In one of the vhosts I added the directives for basic auth (directly into the vhost directive block).
Works like a charm. And I have a better feeling that this is really safe - no risk to overlook any details in the regex pattern that would open up the gates for intruders.
<VirtualHost *:80>
    ServerName www.mysite.com
    DocumentRoot "/path/to/common/doc/root"
    <Directory "/path/to/common/doc/root">
        Options Indexes FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
    </Directory>
</VirtualHost>
<VirtualHost *:80>
    ServerName protected.mysite.com
    DocumentRoot "/path/to/common/doc/root"
    <Directory "/path/to/common/doc/root">
        Options Indexes FollowSymLinks
        AllowOverride All
        Order allow,deny
        Allow from all
        AuthUserFile /path/to/htpasswd
        AuthName "Password please"
        AuthType Basic
        Require valid-user
    </Directory>
</VirtualHost>
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